Tunable devices incorporating cacu3ti4o12

ABSTRACT

This invention provides tunable devices incorporating the dielectric CaCu 3 Ti 4 O 12 . CaCu 3 Ti 4 O 12  is especially useful in tunable devices such as phase shifters, matching networks, oscillators, filters, resonators, and antennas comprising interdigital and trilayer capacitors, coplanar waveguides and microstrips.

FIELD OF THE INVENTION

[0001] This invention provides tunable devices incorporating thedielectric material CaCu₃Ti₄O₁₂.

BACKGROUND OF THE INVENTION

[0002] The use of dielectric materials to increase capacitance is wellknown and long-used. Earlier capacitor dielectrics fell into twocategories. The first category of dielectrics has a relativelytemperature-independent dielectric constant but the value of thedielectric constant is low, e.g., 5-10. Materials such as electricalporcelain and mica fall in this category. The second category ofdielectrics has very high dielectric constant, e.g., 1000 or more, butthey are quite temperature dependent. An example is barium titanate,BaTiO₃.

[0003] Since the capacitance is proportional to the dielectric constant,high dielectric constant materials are desired. In order to performacceptably in tuning or resonance circuits the dielectric must have adielectric constant that exhibits minimal temperature dependence;otherwise small changes in ambient temperature throw the circuit out ofresonance. Other applications require a dielectric constant thatexhibits minimal frequency dependence. It is also desirable to have theloss or dissipation factor as small as possible.

[0004] For many microwave devices the important material features arethe dielectric tunability, i. e., the change in dielectric constant withapplied voltage, and low dielectric loss. Barium strontium titanate,Ba_(1−x)Sr_(x)TiO₃, has been used in some such applications but the needpersists for materials with better properties.

[0005] Deschanvres et al., Bull. Soc. Chim. Fr. 4077 (1967) report thepreparation of CaCu3Ti4O12 with the perovskite structure and a latticeconstant of 0.7393 nm.

[0006] Bochu et al., J. Solid State Chem. 29, 291 (1979) disclose thesynthesis and structure of CaCu3Ti₄O₁₂ and related titanates and reportthe lattice constant to be 0.7391 nm.

[0007] Yandrofski et al., U.S. Pat. No. 5,472,935, disclose tunablemicrowave and millimeter wave devices incorporating tunableferroelectrics.

SUMMARY OF THE INVENTION

[0008] This invention provides tunable devices incorporating thedielectric CaCu₃Ti₄O₁₂. CaCu₃Ti₄O₁₂ is especially useful in tunabledevices such as phase shifters, matching networks, oscillators, filters,resonators, and antennas comprising interdigital and trilayercapacitors, coplanar waveguides and microstrips.

[0009] This invention also provides electrical devices requiring adielectric material with a dielectric constant above 9000 wherein thedielectric is CaCu₃Ti₄O₁₂.

DETAILED DESCRIPTION

[0010] CaCu₃Ti₄O₁₂ has dielectric properties that provide advantages indevices requiring a high dielectric constant as well as in tunabledevices. CaCu₃Ti₄O₁₂ has a dielectric constant above 9000 over afrequency range of 1 kHz to 1 MHz.

[0011] CaCu₃Ti₄O₁₂ can be synthesized by the following procedure.Stoichiometric amounts of the precursors are thoroughly mixed. Theprecursors CaCO₃, CuO and TiO₂ are preferred. The mixed precursor powderis calcined at about 1000° C. for about 12 hours. The calcined powder isreground and pressed to 12.7 mm diameter/1-2 mm thick disks. The disksare sintered in air at about 1100° C. for 24 hours. In both thecalcining and sintering steps, the temperature ramping up rate is about200° C./hour from room temperature, i.e., about 20° C., to the calciningor sintering temperature and the cooling rate is about 150° C./hour fromthe calcining or sintering temperature to room temperature, i.e., about20° C.

[0012] CaCu₃Ti₄O₁₂ crystallizes in a cubic perovskite Im3 structure.

[0013] Dielectric measurements were carried out on the disk samples. Thefaces of the disk-shaped samples were polished with a fine-grit sand oremery paper. Silver paint electrodes were applied on the faces and driedat 70-100° C. The capacitance and the dielectric loss measurements wereperformed by the two-terminal method using Hewlett-Packard 4275A and4284A LCR bridges at a temperature of 25° C. over a frequency range offrom 1 kHz to 1 MHz. The capacitance, C, and the dissipation factor areread directly from the bridge. The dielectric constant (K) wascalculated from the measured capacitance, C in picofarads, from therelationship, K=(100 C t)/(8.854 A), where t is thickness of the diskshaped sample in cm and A is the area of the electrode in cm². Voltageswere applied across the flat electroded faces of the disks andtunability was calculated by measuring the change in dielectric constantwith applied voltage. The tunability, T, is calculated from the equationT=[K(0)−K(V)]/K(0)] where K(0) is the dielectric constant when there isno applied voltage and K(V) is the dielectric constant when there is anapplied voltage V. The tunability is usually expressed as a percent fora given applied electric field so that the above result for T ismultiplied by 100 or it is written as T=(constant) E where T is thetunability in %, E is the electric field and the constant ischaracteristic of the particular material.

EXAMPLE OF THE INVENTION

[0014] CaCu₃Ti₄O₁₂ was prepared by the following procedure. Appropriateamounts of the starting carbonate and oxides CaCO₃, CuO and TiO₂ wereweighed according to the stoichiometric ratios and mixed thoroughly inan agate mortar. The gram amounts of the precursors used are shown inTable 1. The mixed powder was calcined at 1000° C. for 12 hours. Thecalcined powder was reground and pressed to 12.7 mm diameter/1-2 mmthick disks. The disks were sintered in air at 1100° C. for 24 hours. Inboth the calcining and sintering steps, the temperature was increasedfrom room temperature, i.e., about 20° C., to the calcining or sinteringtemperature at a rate of 200° C./hour and the temperature was decreasedfrom the calcining or sintering temperature to room temperature, i.e.,about 20° C., at a rate of 150° C./hour.

[0015] X-ray powder diffraction patterns were recorded with a SiemensD5000 diffractometer. The data showed that CaCu₃Ti₄O₁₂ crystallized in acubic perovskite related Im3 structure. The measured lattice parameterand the literature value are listed in Table 1. TABLE 1 CaCO₃ 0.2502 gCuO 0.5966 g TiO₂ 0.799 g a, measured .7391(1) nm lattice parameter a,literature value .7391 nm lattice parameter

[0016] The disk samples were polished to produce flat uniform surfacesand electroded with silver paint. The painted samples were dried at70-100° C. overnight. Capacitance and loss tangent measurements weretaken on a

[0017] HP-4284A LCR meter at room temperature, i.e., about 20° C., overa frequency range of from 1 kHz to 1 MHz. The results are shown in Table2. TABLE 2 Frequency Dielectric Loss (Hz) Constant Tangent 1 k 123340.105 10 k 11087 0.0695 100 k 10286 0.0674 1 M 9211 0.232

[0018] Voltages up to 100V were applied across the flat electroded facesof the disks using an Keithley 228A voltage/current source and thedielectric constant was measured as a function of applied voltage atroom temperature using a HP-4275A LCR meter. The percent tunability andthe applied electric filed to obtain that magnitude tunability are shownin Table 3 over a frequency range of from 1 kHz to 1 MHz. The tunabilityequation written in the form T=(constant) E is also given in Table 3 foreach frequency. TABLE 3 Electric Frequency Field (V/μm) 10 k 100 k 1 M10 M 0 0.000 0.000 0.000 0.000 0.017182 1.149 1.015 0.805 0.328 0.0343642.546 2.258 1.849 0.786 0.051546 3.924 3.491 2.926 1.290 0.068729 5.2444.676 3.896 1.841 0.081615 6.201 5.536 4.627 2.182 Tunability T = 77.0 ×E T = 68.8 × E T = 57.7 × E T = 27.5 × E

[0019] The results show that CaCu₃Ti₄O₁₂ has a high tunability alongwith a high dielectric constant.

What is claimed is:
 1. A tunable electric device comprising CaCu₃Ti₄O₁₂.2. A tunable electrical device requiring a dielectric material with adielectric constant above 9000 comprising the dielectric CaCu₃Ti₄O₁₂.